The object of the present invention is a nozzle arrangement in an airborne web-drying apparatus and a method for improving the heat transfer in airborne web-drying, the apparatus and the method being defined in the preambles of the independent claims presented below.
Then the object of the invention is typically a nozzle arrangement which comprises at least one overpressure nozzle extending transversely of the web and having on both sides of the nozzle, i.e. on the entrance and exit sides of the nozzle, a nozzle slot extending across the web, in which case the nozzle slots on the opposite sides of the nozzle comprise one nozzle slot extending across the web or a row of successive nozzle orifices. The nozzle slots are arranged to blow drying air jets obliquely against each other, or they are arranged to blow drying air jets, which are guided against each other with the aid of curved Coanda-surfaces. The arrangement further comprises at least one direct impingement nozzle extending across the web, in which case a plurality of nozzle slots or nozzle orifices are formed in this direct impingement nozzle for blowing drying air mainly perpendicularly against the web. Advantageously the nozzle orifices or slots of the direct impingement nozzle are arranged in one or more rows, or otherwise evenly distributed on the supporting surface of the direct impingement nozzle.
A plurality of overpressure nozzles or direct impingement nozzles are typically arranged in an alternating succession on both sides of the web. Thereby an overpressure nozzle and a direct impingement nozzle are arranged opposite each other, as shown e.g. in the international patent publication WO 95/14199. In the solution presented in the WO-publication the space between each overpressure nozzle and the adjacent direct impingement nozzle forms a discharge passage for the wet discharge air. The discharge passages are ineffective regions regarding the drying of the web.
The aim is to continuously improve the effect of the airborne web-drying for instance in order to be able to make the drying faster and/or to reduce the size of the dryer. One economical means to improve the effect of airborne web-drying is to increase the nozzle temperature. However, it is not possible to increase the nozzle temperature in some applications, or the desired effect can not be obtained with this single measure.
The object of the present invention is to provide an improved nozzle arrangement and a method which are able to increase the effect of airborne web-drying.
A particular object is to provide a nozzle arrangement which is easy to realise in airborne web-drying apparatuses of different types.
A further object is to provide an improved nozzle arrangement and method which do not require substantial extra space for the airborne web-drying apparatus.
In order to reach the above-mentioned objects the nozzle arrangement and method according to the invention in airborne web-drying are characterised in what is defined in the characterising parts of the independent claims presented below.
The solution according to the invention uses nozzle assemblies which in the same structure combine at least one overpressure nozzle and at least one direct impingement nozzle. The assembly of overpressure nozzle and direct impingement nozzle is advantageously mounted in a common frame structure and in a common nozzle box. The nozzle assembly comprises typically an overpressure nozzle and a direct impingement nozzle arranged on both sides of the overpressure nozzle, i.e. on its entrance and exit sides. Thus no conventional discharge passage for wet air is formed between the overpressure nozzle and the direct impingement nozzles in the nozzle assembly. Compared to conventional solutions a larger part of the area of the dryer can in this way be utilised in the actual drying process. The discharge passages for the wet air are arranged between the different nozzle assemblies. Each passage discharges drying air blown by both the overpressure nozzle and the direct impingement nozzle. The direct impingement nozzles are arranged in relation to the web, so that they do not hinder air from being discharged from the overpressure nozzle. The web will further facilitate the air discharge from the direct impingement nozzle region in the travel direction of the web.
In another typical solution according to the invention a direct impingement nozzle is arranged on the entrance or exit side in the travel direction of the web of the over pressure nozzle and directly attached to the overpressure nozzle, so that an assembly comprising an overpressure nozzle and one direct impingement nozzle is formed.
The distance between the nozzle slots of the overpressure nozzle and the first nozzle orifice row closest to the overpressure nozzle is advantageously  greater than 30 mm but  less than 100 mm, typically 40 to 60 mm.
In conventional airborne web-drying solutions there is a relatively wide discharge air passage between each successive nozzle pair. Then the actual nozzles cover only less than half of the total area. In this case there will be a poor heat transfer in the region of the discharge air passage, as no air jets are directed at the web in this region. In the solution according to the invention the drying utilises also a part of the empty space left between the individual nozzles in conventional dryers. The direct impingement arranged in connection with the overpressure nozzle enables an increased total amount of drying air to be directed at the web, i.e. in this region the heat-transfer coefficient can be increased and the heat transfer can be made more efficient. In measurements it was found that a considerably increased heat transfer can be achieved with the solution according to the invention. The heat transfer can be made more efficient with the solution according to the invention, also when the temperature of the drying air must kept very low, such as for instance in the drying of xe2x80x9cthermal coatingsxe2x80x9d.
Each nozzle assembly according to the invention has typically nozzle orifices in one or two direct impingement nozzle sections, the nozzle orifices occupy an area having a total length of 20 to 250 mm in the travel direction of the web, typically  greater than 50 mm, most typically  greater than 100 mm, or covering 10 to 60% of the length of the nozzle distribution. A direct impingement nozzle can of course also have only one row of nozzles or nozzle orifices, in which case the area is very small.
The nozzle orifices of the direct impingement nozzle parts have typically a diameter of 2 to 10 mm, most typically about 5 mm, and the nozzles are arranged at a distance from each other which is 10 to 50 mm, typically 20 to 30 mm, both in the web cross direction and in the web travel direction. The nozzle orifices are typically arranged in rows in the cross direction of the web. There are typically 2 to 7 successive rows of nozzle orifices in the travel direction of the web. Advantageously the nozzle orifices in different rows are overlapping, so that the total coverage of the orifices is as large as possible. The nozzle orifices can also be arranged evenly on the supporting surface of the nozzle in other ways. An airborne web-drying apparatus contains typically several successive nozzle assemblies on both sides of the web to be dried.
In steam-heated dryers the heat source forms an upper limit for the temperature. Also in this case the drying can be made more effective with the solution according to the invention. An effective nozzle can increase the drying effect also in gas-heated dryers.
On the other hand the solution according to the invention can also be used in small spaces, particularly in short spaces, in order to maximise the drying effect.
The gap between two successive assemblies according to the invention forms a discharge passage for wet discharge air. The nozzle assemblies are disposed on different sides of the web to be dried, advantageously in such a manner that there is always a part of a nozzle assembly, preferably an overpressure nozzle part, on the other side of the web opposite to a discharge passage. The intention is to avoid a situation where two discharge passages would be located opposite each other. The aim is that the web is guided at all points by drying air blows, at least from one side of the web. An aim is also usually to arrange the overpressure nozzles in the airborne web-drying apparatus so that they cause the web to travel forward like a sine wave.
In an advantageous nozzle arrangement solution according to the invention the nozzle surface of the direct impingement nozzle, i.e. the supporting surface of the nozzle, is at a longer perpendicular distance from the web line than the overpressure nozzle. The web line means typically a straight line located centrally between the drying boxes on opposite sides of the web. The web itself travels along the web line, but however, often like a sine wave. The distance of the nozzle surface of a direct impingement nozzle from the web line is advantageously 5 to 40 mm, typically 10 to 15 mm, longer than the distance of the supporting surface of an overpressure nozzle from the web line. The perpendicular distance of the nozzle surface of a direct impingement nozzle from the web line is typically about 20 to 30 mm. This ensures a discharge gas space on the entrance and exit sides of the nozzle between the direct impingement nozzle and the web, for air blown from the nozzle slots on the entrance and exit sides of the overpressure nozzle.
When the nozzle surface of the direct impingement nozzle is located at a greater distance from the web line than the nozzle surface or the supporting surface of the overpressure nozzle, it is guaranteed that the air jets from the direct impingement nozzle part do not interfere with the operation of the overpressure nozzle. Preferably the structure of the direct impingement nozzle and its air jets must be dimensioned, so that the air jets turn suitably away from the overpressure nozzle toward, the discharge passage of the return air, i.e. the discharge air, and do not tend to form an obstruction to the air flow leaving the overpressure nozzle.
The discharge passage between two adjacent nozzle assemblies is advantageously dimensioned so that it can remove, regarding the travel direction of the web
the discharge air from the exit side of the overpressure nozzle on the upstream side of the discharge passage, and the discharge air from the direct impingement nozzle arranged on the exit side of this overpressure nozzle, and
the discharge air from the entrance side of the overpressure nozzle on the downstream side of the discharge passage, and the discharge air from the direct impingement nozzle arranged on the entrance side of this overpressure nozzle.
The area of the discharge passage in the web direction is advantageously less than 40% of the corresponding total area of the airborne web-drying apparatus, i.e. of the corresponding area covered by the nozzles and the discharge passage.
The total area (A1) of the openings of the direct impingement nozzle or nozzles in each direct impingement nozzle and overpressure nozzle assembly is typically
about 40 to 100% of the total area (A2) of the nozzle slots of the overpressure nozzle when there is a direct impingement nozzle only on one side, and
about 40 to 150% of the total area (A2) of the nozzle slots of the overpressure nozzle when there is a direct impingement nozzle on both sides of the overpressure nozzle.
The width of the nozzle slots of the overpressure nozzles is typically about 1.5 mm. The open area of the slots of the overpressure nozzles is 1 to 2%, typically 0.8 to 1.5%, most typically about 1.2% of the total area of the airborne web-drying apparatus. The open area of the orifices of the direct impingement nozzles is correspondingly about 0.5 to 1.5% of the total area of the airborne web-drying apparatus. Sometimes smaller or larger opening areas can come into question.
In some cases, particularly when the width of the direct impingement nozzle in the web travel direction is relatively large, the nozzle surface of the direct impingement nozzle arranged on the exit side of the overpressure nozzle can be curved, so that its distance from the web increases in the travel direction of the web.
With the method according to the invention the heat transfer in airborne web-drying can be effectively increased by blowing drying air directly on the exit and/or entrance side of the overpressure nozzle, mainly perpendicularly against the web, with the aid of a direct impingement nozzle having the nozzle surface at a larger distance from the web than the nozzle surface of the overpressure nozzle. Thus the solution according to the invention ensures that the drying air blown from the nozzle slots on the exit side and/or the entrance side of the overpressure nozzle and the drying air blown from the direct impingement nozzle form wet discharge air, which can be guided away from the web region via a discharge passage formed on the exit side and/or entrance side of the direct impingement nozzle, without interfering with the operation of the overpressure nozzle.